Though our present day rivals Noah's in corruption and evil, this meaning of Matthew 24:36-39 is actually the secondary interpretation. The primary meaning is more simple: Christ would come at a time when most of the world was busy doing its normal activities.
Notice verse 36: "But of that day and hour no one knows, no, not even the angels of heaven, but My Father only." This is the subject sentence of the entire paragraph. Verse 42 repeats the thought: "Watch therefore, for you do not know what hour your Lord is coming." Jesus rephrases it in verse 44: "Therefore you also be ready, for the Son of Man is coming at an hour when you do not expect Him."
Luke's version makes this especially clear:
And as it was in the days of Noah, so it will be in the days of the Son of Man: They ate, they drank, they married wives, they were given in marriage, until the day that Noah entered the ark, and the flood came and destroyed them all. Likewise as it was also in the days of Lot: They ate, they drank, they bought, they sold, they planted, they built; but on the day that Lot went out of Sodom it rained fire and brimstone from heaven and destroyed them all. Even so will it be in the day when the Son of Man is revealed. (Luke 17:26-30)
People will be involved in their normal activities, not realizing such a momentous event is about to occur!
Paul writes that "the day of the Lord so comes as a thief in the night. For when they say, 'Peace and safety!' then sudden destruction comes upon them, as labor pains upon a pregnant woman. And they shall not escape" (I Thessalonians 5:2-3). Just when men begin to think they have a handle on society's problems, total chaos and destruction will erupt.
Peter reminds us of scoffers coming in the last days who would say, "Where is the promise of His coming? For since the fathers fell asleep, all things continue as they were from the beginning of creation" (II Peter 3:4). The apostle goes on to cite the example of the Flood—which came on suddenly and unexpectedly—as an event that broke the natural cycle of life (verses 5-6). Such is the time of Christ's return.
All of these prophetic warnings include the admonition to watch and be ready for it when it comes. As Paul says:
But you, brethren, are not in darkness, so that this Day should overtake you as a thief. . . . Therefore let us not sleep, as others do, but let us watch and be sober, . . . putting on the breastplate of faith and love, and as a helmet the hope of salvation. (I Thessalonians 5:4, 6, 8)
Christ's return will not be sudden and unexpected to the church. We may not know the day or the hour, but we will be somewhere "in the ballpark." Jesus says His day "will come as a snare on all those who dwell on the face of the whole earth" (Luke 21:35). But, as verse 34 says, if we "take heed to [our]selves," not being mired down by sin, we will be expecting it.
We are certainly living in times very like those of the days of Noah, so the return of Christ could come anytime soon. Knowing this, our job is to watch and pray and overcome so "that you may be counted worthy to escape all these things that will come to pass, and to stand before the Son of Man" (verse 36).